Backlights are used to illuminate liquid crystal displays (“LCDs”). LCDs with backlights are used in small displays for cell phones and personal digital assistants (“PDAs”) as well as in large displays for computer monitors and televisions. Often, the light source for the backlight includes one or more cold cathode fluorescent lamps (“CCFLs”). The light source for the backlight can also be an incandescent light bulb, an electroluminescent panel (“ELP”), or one or more hot cathode fluorescent lamps (“HCFLs”).
The display industry is enthusiastically pursuing the use of LEDs as the light source in the backlight technology because CCFLs have many shortcomings: they do not easily ignite in cold temperatures, they require adequate idle time to ignite, and they require delicate handling. Moreover, LEDs generally have a higher ratio of light generated to power consumed than the other backlight sources. Because of this, displays with LED backlights can consume less power than other displays. LED backlighting has traditionally been used in small, inexpensive LCD panels. However, LED backlighting is becoming more common in large displays such as those used for computers and televisions. In large displays, multiple LEDs are required to provide adequate backlight for the LCD display.
Circuits for driving multiple LEDs in large displays are typically arranged with LEDs distributed in multiple strings. FIG. 1 shows an exemplary flat panel display 10 with a backlighting system having three independent strings of LEDs 1, 2 and 3. The first string of LEDs 1 includes seven LEDs 4, 5, 6, 7, 8, 9 and 11 discretely scattered across the display 10 and connected in series. The first string 1 is controlled by the drive circuit 12. The second string 2 is controlled by the drive circuit 13 and the third string 3 is controlled by the drive circuit 14. The LEDs of the LED strings 1, 2 and 3 can be connected in series by wires, traces or other connecting elements.
FIG. 2 shows another exemplary flat panel display 20 with a backlighting system having three independent strings of LEDs 21, 22 and 23. In this embodiment, the strings 21, 22 and 23 are arranged in a vertical fashion. The three strings 21, 22 and 23 are parallel to each other. The first string 21 includes seven LEDs 24, 25, 26, 27, 28, 29 and 31 connected in series, and is controlled by the drive circuit, or driver, 32. The second string 22 is controlled by the drive circuit 33 and the third string 23 is controlled by the drive circuit 34. One of ordinary skill in the art will appreciate that the LED strings can also be arranged in a horizontal fashion or in another configuration.
An important feature for displays is the ability to control the brightness. In LCDs, the brightness is controlled by changing the intensity of the backlight. The intensity of an LED, or luminosity, is a function of the current flowing through the LED. FIG. 3 shows a representative plot of luminous intensity as a function of forward current for an LED. As the current in the LED increases, the intensity of the light produced by the LED increases.
To generate a stable current, circuits for driving LEDs use constant current sources. FIG. 4 is a representation of a circuit used to generate a constant current. A constant current source is a source that maintains current at a constant level irrespective of changes in the drive voltage VSET. Constant current sources are used in a wide variety of applications; the description of applications of constant current sources as used in LED arrays is only illustrative. The operational amplifier 40 of FIG. 4 has a non-inverting input 41, an inverting input 42, and an output 43. To create a constant current source, the output of the amplifier 40 may be connected to the gate of a transistor 44. The transistor 44 is shown in FIG. 4 as a field effect transistors (“FET”), but other types of transistors may be used as well. Examples of transistors include IGBTs, nMOS devices, JFETs and bipolar devices. The drain of the transistor is connected to the load 45, which in FIG. 4 is an array of LEDs. The inverting input of the amplifier 40 is connected to the source of the transistor 44. The source of the transistor 44 is also connected to ground through a sensing resistor RS 46. When a reference voltage, is applied to the non-inverting input of the amplifier 40, the amplifier increases the output voltage until the voltage at the inverting input matches the voltage at the non-inverting input. As the voltage at the output of the amplifier 40 increases, the voltage at the gate of the transistor 44 increases. As the voltage at the gate of the transistor 44 increases, the current from the drain to the source of the transistor 44 increases.
For an LED backlit display to operate at a given brightness, the current in the drain current of the transistor 44 must be maintained at a set level: the design current. The design current may be a fixed value or it may change depending upon the brightness settings of the display.
FIG. 5 illustrates a typical relationship between the drain current and the gate voltage for an exemplary transistor. Since little to no current flows into the inverting input of the amplifier 40, the increased current passes through the sensing resistor RS. As the current across the sensing resistor RS increases, the voltage drop across the sensing resistor also increases according to Ohm's law: voltage drop (V)=current (i)*resistance (R). This process continues until the voltage at the inverting input of the amplifier 40 equals the voltage at the non-inverting input. If, however, the voltage at the inverting input is higher than that at the non-inverting input, the voltage at the output of the amplifier 40 decreases. That in turn decreases the source voltage of the transistor 44 and hence decreases the current that passes from the drain to the source of the transistor 44. Therefore, the circuit of FIG. 4 keeps the voltage at the inverting input and the source side of the transistor 44 equal to the voltage applied to the non-inverting input of the amplifier 40 irrespective of changes in the drive voltage VSET.
Large displays with LED backlights use multiple constant current sources like that of FIG. 4. Therefore, large LED-backlit displays use many transistors 44. Transistors are limited in the maximum drain-to-source voltage and drain current that the transistor can safely handle. Curves that show a transistor's limitations of simultaneous high voltage and high current, up to the rating of the device, are often provided to circuit designers by transistor manufacturers. These curves are generally known as safe operating area curves. The safe operating area (“SOA”) of the transistor is the area below the curve. An example of an SOA curve is shown in FIG. 6.
FIG. 6 illustrates a SOA curves for two different operating conditions: continuous current mode 60 and discontinuous pulse current mode 61. Multiple SOA curves for discontinuous pulse current modes 61 based upon the relative pulse duration are generally provided by the transistor manufacturer. For a given forward drain current, the SOA curve instructs circuit designers on the maximum drain-to-source voltage that the transistor can safely handle. For example, at the continuous drain current 62 in FIG. 6, the maximum safe drain-to source voltage 63 for the transistor is determined from the SOA curve. If the maximum safe drain-to-source voltage 63 is exceeded at the drain current 62 shown, the transistor is at risk of failure or degradation. Therefore, circuit designers must ensure the operation of the transistor is within its SOA.
To expand the area under the SOA curve for higher maximum drain current ratings, the size of the transistor must be increased. Larger transistors are more expensive and require a larger die size if integrated into a single die or integrated circuit. To extend the area under the SOA curve for higher maximum drain-to-source voltages, an enhanced or more complex fabrication process must be used. Transistors fabricated for larger drain-to-source voltages might not be readily available or cost effective for many designs. To reduce device size and costs, circuit designers often choose the basic minimum-geometry transistor that can safely operate at the design drain-to-source voltage and design drain current. However, this often limits the available overhead room for increased drain-to-source voltage at the design drain current.
Occasionally, the drain-to-source voltage of the transistor 44 may unexpectedly increase above the design level. This may happen because of inadvertent over-voltage of the drive voltage VSET or due to shorting of the load 45. Shorting of the load 45 can happen for many reasons including foreign material shorting the load path, improper soldering during assembly of the circuit, and damage in the load. When the drain-to-source voltage increases from the design voltage due to a short, it may increase all the way to the drive voltage VSET. When the drain-to-source voltage inadvertently increases at a given drain current, the operating point of the transistor may go beyond the safe operating area. An example of this for a transistor operated in continuous current mode is shown at point 64 in FIG. 6. At point 64, the drain-to-source voltage has increased to the drive voltage VSET. The drain current is at the design current 62. Since the operating condition 64 of the transistor is outside of the safe operating area, the transistor has a high probability of immediate failure or degradation. If a transistor fails or degrades, the current source will no longer function properly. Transistor failure or degradation causes safety and reliability problems and therefore increases recall and warranty costs for device manufacturers.
For a circuit that could safely operate at the design current 62 and drain-to-source voltage VSET, circuit designers would have to use a much larger transistor with a SOA that encompassed the point defined by the design current 62 and drain-to-source voltage VSET. A larger transistor would be more expensive and more difficult to integrate into a device designed to be integrated into a single chip.